This invention relates to a method for the production of a porous cross-linked polymer material by the use of a polyglycerine fatty acid ester as a surfactant.
As a technique for obtaining a porous substance formed of uniform open cells of a minute diameter, a method for obtaining the polymer in a water-in-oil type high internal phase emulsion (hereinafter referred to briefly as xe2x80x9cHIPExe2x80x9d) in the presence of a specific surfactant has been known. It is generally held that the term xe2x80x9chigh internal phase emulsionxe2x80x9d as used herein means an emulsion in which the ratio of the internal phase to the total volume of the emulsion exceeds 70 vol. % [K. J. Lissant, Journal of Colloid and Interface Science, Vol. 22, 462 (1966)]. U.S. Pat. No. 5,334,621, for example, discloses a method for producing a porous material by cross-link polymerizing a polymerizing monomer contained in the HIPE (hereinafter referred to the xe2x80x9cHIPE methodxe2x80x9d).
This HIPE method produces a porous cross-linked polymer by preparing an HIPE containing (i) a polymerizing monomer mixture containing an oil-soluble vinyl monomer and a cross-linking monomer possessed of not less than two functional group in the molecular unit, (ii) a water phase accounting for 90 wt. %, preferably 95 wt. %, and particularly preferably 97 wt. % of the emulsion, (iii) a surfactant active agent such as a sorbitan fatty acid ester and a glycerol mono-fatty acid ester, and (iv) a polymerization initiator and heating the HIPE till it is polymerized and cross-linked. Generally, the porous cross-linked polymer is produced by mixing the oil phase containing at least the components (i) and (iii) mentioned above and the water phase, namely the component (ii), and emulsifying the resultant mixture thereby preparing an HIPE and adding an initiator, namely the component (iv), to the HIPE and, at the same time, heating them together to a temperature most suitable for the polymerization of the HIPE and initiating this polymerization. According to this HIPE method, since a porous material formed of open cells in a reticular pattern by a reversed-phase emulsion polymerization, the produced porous material is fated to acquire such characteristic properties as low density, water absorbing property, water retaining property, heat insulating property, and sound insulating property.
As a technique for preparing such an HIPE, the official gazette of WO97/45456, for example, discloses a method for preparing this HIPE by emulsification using at least one sorbitan fatty acid ester or sugar fatty acid ester with a view to stabilizing the HIPE. Though it has no mention of the temperature for the preparation of the HIPE, it has a statement to the effect that when the HIPE is in a stable range, it can be cured, namely polymerized, at a temperature in the range of 25-90xc2x0 C. In fact, when sorbitan monolaurate and ditallow dimethyl ammonium chloride are used in combination, an HIPE is prepared by using a water phase at 40xc2x0 C. and the HIPE is polymerized at 70xc2x0 C. for 24 hours to produce a polymer material.
The official gazette of WO97/45479 discloses a method for preparing an HIPE by using an emulsifier system composed of an anionic surfactant possessed of an oleophilic moiety and an anion group and a quaternary salt possessed of a hydrocarbon group of not less than eight carbon atoms. Though it has no mention of the temperature for preparing the HIPE, it has a statement to the effect that the HIPE can be cured, namely polymerized, at least at a temperature of not less than 25xc2x0 C.
The HIPE method, as described above, obtains a porous cross-linked polymer by emulsifying an oil phase containing at least one species of polymerizing monomer and a water phase by the use of an emulsifier such as a surfactant and then polymerizing the resultant HIPE by means of heating. Where this process is performed continuously, it is necessary for the purpose of exalting the efficiency of production to shorten the time for preparing an HIPE and the time for subsequently polymerizing the HIPE. Generally the HIPE is prepared in an emulsifying device and then the HIPE is polymerized in a polymerizing device. Where the polymerization of the HIPE is initiated by adding a polymerization initiator to the HIPE and subsequently heating the HIPE containing the polymerization initiator, therefore, the fact that the temperature of the HIPE in the emulsifying device and the polymerization temperature of the HIPE in the polymerizing device are approximated closely to each other is believed to bring the advantage of curtaining the time for initiating the polymerization of the HIPE.
Since the HIPE generally contains such a large amount of water that the water content thereof reaches a level in the range of 1,000-25,000 (v/v) %, however, it is extremely deficient in stability and is deprived of stability even by a slight increase of temperature. Normally, therefore, the practice of preparing an HIPE by emulsifying an oil phase containing in advance therein a polymerization initiator and a water phase at a temperature in the range of 25-40xc2x0 C. has been in vogue. Such is the true state of affairs.
Further, when a porous cross-linked polymer is produced by polymerizing an HIPE, the charged raw materials possibly succumb to degeneration and consequently emit offensive odor. Since the degeneration of this sort depends particularly on temperature, it is naturally preferable to prepare the HIPE at a low temperature for the purpose of preventing the components used in the HIPE from generating to such degeneration. In consideration of the instability of the HIPE due to elevation of temperature as a contributory factor, the preparation of the HIPE inclusive of the preparation of the oil phase for the production of the HIPE and the preparation of the water phase is generally required to be carried out in the neighborhood of normal room temperature.
In this respect, the inventions disclosed in the official gazette of WO97/45456 and the official gazette of WO97/45479 have been aimed at stabilizing an HIPE and fulfilling the polymerization of an HIPE at a high temperature. The stabilization attained by the inventions does not deserve to be rated as fully satisfactory because their methods actually degrade the emulsifying power during the steps of emulsification and polymerization and induce separation of water. Particularly, the method taught in the official gazette of WO97/45456, when expected to attain the emulsification by using a water phase heated at a temperature higher than 40xc2x0 C., has the possibility of degrading the HIPE in stability, entailing separation of a large amount of water during the course of emulsification or polymerization, and inducing the polymer material to suffer from degradation of performance.
The produced porous cross-linked polymer can be used as a sound insulating material or a heat insulating material for the purpose of absorbing sound or heat, as a chemical impregnating substrate for the purpose of imbibing an aromatic material or a detergent, and as an aborbing material for oil or organic solvent. It can also be used as a sanitary material as in disposable diapers or sanitary articles or as a cosmetic or medical material for applications not fated to create direct contact with the human system. In this case, the surface of the porous cross-linked material as a finished product is required to reduce the stimulus to the skin to the fullest possible extent for the purpose of preventing the user from forming skin rash or experiencing unpleasant feeling. When the charged raw materials are caused as by degeneration to emit offensive odor or produce stimulus, the offensive odor or the stimulus possibly persists in the product. An effort to remove it from the product gives rise to the necessity of fortifying the washing step. Since the HIPE inherently has a water content in the range of 1,000-25,000 (v/v) % and the preparation of the HIPE itself requires a large amount of water to be used, it is necessary to decrease the washing water to be used to the fullest possible extent. Particularly, the porous cross-linked polymer has a large surface area for contact with the ambient air and, therefore, requires a large amount of washing water to be thoroughly cleaned. This fact implies the disposal and expulsion of a large amount of waste water and brings such adverse effects as increasing the cost of production and augmenting the load on the environment.
The present inventor has pursued a diligent study with a view to developing a method for producing a porous cross-linked polymer material in a very short period of time in accordance with the principle of the HIPE method. It was consequently found that when a polyglycerine fatty acid ester is used as a surfactant and a consequently obtained HIPE is polymerized at a high temperature, a porous cross-linked polymer material suffering separation of waster only in small amount is obtained in a short time and a finally obtained porous cross-linked polymer material excels in absorption properties. This invention has been perfected based on this discovery. Specifically, this invention is aimed at providing the following item (1).
(1) In producing a porous cross-linked polymer by forming an HIPE and subsequently polymerizing the HIPE, a method for the production of a porous cross-linked polymer material which comprises a step of polymerizing the HIPE obtained in the presence of a polyglycerine fatty acid ester.
According to this invention, the HIPE can be polymerized in such a heretofore unforeseeable short span of time as not more than one hour, preferably not more than 30 minutes and the porous cross-linked polymer material excelling in absorbing properties can be efficiently produced.